Existing microwave antennas include a wide variety of configurations for various applications, such as satellite reception, remote broadcasting, or military communication. The desirable characteristics of low cost, light weight, low profile and mass producibility are provided in general by printed circuit antennas. The simplest forms of printed circuit antennas are microstrip antennas wherein flat conductive elements, such as monopole or dipole antenna elements, are spaced from a single essentially continuous ground plane by a dielectric sheet of uniform thickness. An example of a microstrip antenna is disclosed in U.S. Pat. No. 3,995,277 to Olyphant.
The antennas are designed in an array and may be used for communication systems such as identification of friend/foe (IFF) systems, personal communication service (PCS) systems, satellite communication systems, and aerospace systems, which require such characteristics as low cost, light weight, low profile, and a low sidelobe. The bandwidth and directivity capabilities of such antennas, however, can be limiting for certain applications.
The use of electromagnetically coupled dipole antenna elements can increase bandwidth. Also, the use of an array of dipole antenna elements can improve directivity by providing a predetermined maximum scan angle.
However, utilizing an array of dipole antenna elements presents a dilemma. The maximum grating lobe free scan angle can be increased if the dipole antenna elements are spaced closer together, but a closer spacing can increase undesirable coupling between the elements, thereby degrading performance. This undesirable coupling changes rapidly as the frequency varies, making it difficult to maintain a wide bandwidth.
One approach for compensating the undesirable coupling between dipole antenna elements is disclosed in U.S. Pat. No. 6,417,813 to Durham, which is incorporated herein by reference in its entirety and which is assigned to the current assignee of the present invention. The Durham patent discloses a wideband phased array antenna comprising an array of dipole antenna elements, with each dipole antenna element comprising a medial feed portion and a pair of legs extending outwardly therefrom.
In particular, adjacent legs of adjacent dipole antenna elements include respective spaced apart end portions having predetermined shapes and relative positioning to provide increased capacitive coupling between the adjacent dipole antenna elements. The increased capacitive coupling counters the inherent inductance of the closely spaced dipole antenna elements, in such a manner as the frequency varies so that a wide bandwidth may be maintained.
The number of elements in an array of dipole antenna elements may range from several hundred to several thousand, with all of these elements being on the same substrate surface. To provide a uniform driving point impedance for the active dipole antenna elements along the edges of the array (i.e., the impedance for the elements along the edges is the same or very close to that of any element near the center of the array), dummy elements, which do not transmit or receive signals, are placed adjacent these elements.
However, design constraints for certain applications may limit the array size so that it has a significantly reduced number of active dipole antenna elements. For example, a small array of 50 elements with dummy dipole antenna elements along the edges thereof results in the percentage of dummy dipole antenna elements being large (>60%) as compared to the percentage of active dipole antenna elements (<40%) that actually transmit and receive signals. Consequently, performance of the phased array antenna is reduced, gain would be lower, and the beamwidth would be broader because of the area that is to be made available for the dummy dipole antenna elements on the same substrate as the active dipole antenna elements.
One approach for providing a uniform impedance for the active dipole antenna elements along the edges of the array while increasing performance is disclosed in U.S. Pat. No. 6,448,937 to Aiken et al. In Aiken et al., the dummy dipole antenna elements are fed separately from the active dipole antenna elements so that they are also able to transmit and receive signals. These separately fed elements also provide a uniform impedance for the active dipole antenna elements along the edges of the array. However, the additional feed lines for the dummy dipole antenna elements increase the complexity of the phased array antenna.